1. Field of the Invention
The present invention relates to a marine propulsion system. In particular, the present invention relates to a lower unit design of an outboard drive.
2. Description of Related Art
Lower units of outboard motors commonly house a transmission or a like drive transfer mechanism beneath the water surface of the body of water in which the outboard motor is operated. The drive transfer mechanism in the lower unit transfers power from a drive shaft of the outboard motor to at least one propulsion shaft. The propulsion shaft in turn drives a propulsion device, such as, for example, a propeller.
Prior lower units generally include a housing or nacelle which houses the drive transfer mechanism. The nacelle generally has a cone-like shape with a blunt, rounded nose (i.e., generally has a bullet-like shape). Such prior configurations present a streamline shape when the axis of the propeller shaft lies generally parallel to the direction of water flow over the lower unit. The streamline shape inhibits boundary layer separation within the water stream over the lower unit to minimize water resistance (i.e., drag) on the lower unit.
The lower unit, however, desirably does not remain in one position for all operation conditions. Rather, the position of the propeller shaft relative to the horizontal (i.e., the trim angle) desirably changes depending upon the running condition of the watercraft. For instance, when running at high speed, the propeller is trimmed up (i.e., "trimmed out," away from the watercraft transom) to position the propeller shaft at a positive trim angle relative to the horizontal. The positive trim position of the propeller shaft maintains the planing condition of the watercraft with the bow of the watercraft riding out of the water.